ABSTRACT Population-level interventions to encourage healthy eating?which involve changing food environments or purchasing conditions?have been extensively discussed for the prevention of obesity and type 2 diabetes. Preliminary data suggest that population interventions introduced in isolation (such as an SSB tax) can have a modest effect, but multi-faceted interventions should be introduced in combination, to produce large and meaningful improvements in healthy eating. Studying combinations of population interventions is difficult, as such interventions are rare. We propose to rigorously assess a rare set of nested natural experiments in the two largest, most diverse urban settings in California (CA): Los Angeles and the San Francisco Bay Area. We will identify the complementarities among major population-level interventions to encourage healthy eating. In Aim 1, we will conduct a prospective evaluation of a population-wide SSB tax in both tax-affected and matched unaffected CA counties. We will identify changes to both beverage and food consumption and purchasing, food substitution patterns, spillover to neighboring areas, effects of marketing, and any regressive economic effects for low-income populations. The longer-term follow-up period enabled by our design will test the hypothesis that a SSB tax produces sustained declines in SSB consumption rather than merely short-term effects followed by a return to pre-tax consumption. Uniquely, we will then test complementarity between the tax and other interventions in Aims 2 and 3. In Aim 2, we will examine the complementarity of the SSB tax with a fruit and vegetable voucher program. Some SSB tax-affected and distant tax-unaffected counties have introduced fresh fruit and vegetable purchasing vouchers, which enable low-income families facing food insecurity to purchase fruits and vegetables at major supermarkets and corner stores in the area, not just at farmer?s markets. We will test the hypothesis, supported by our preliminary data, that pairing the purchasing penalty of an SSB tax with a purchasing incentive of a fruit and vegetable voucher improves upon the modest effect of either intervention alone. In Aim 3, we will identify the complementarity between the SSB tax and a workplace SSB purchasing ban, implemented in some workplaces but not others during tax-affected and -unaffected periods. We will test the hypothesis, consistent with our preliminary data, that workplace bans disproportionately reduce SSB consumption in the lowest-income employees, neutralizing the economically-regressive effect of a SSB tax. We will evaluate these interventions using innovative methods that advance the study of natural experiments in obesity and diabetes prevention. Specifically, we will use a synthetic control statistical method, which adjusts for unmeasured confounders such as differences in culture among the intervention and control areas. Second, we will input our results into validated computer simulation models of obesity, type 2 diabetes, and cardiovascular disease, allowing us to project the generalizable impact and cost-effectiveness of the interventions for reducing long-term national race/ethnic and socioeconomic disparities.